The present invention is concerned with improving the operating characteristics of peristaltic pumps, more particularly, by regulating the operation of the pump drive motor. Whilst it is specifically directed to improving the operation of peristaltic medical infusion pumps, is is equally applicable to other peristaltic pumps where volumetric accuracy and constancy or uniformity of flow are important requirements.
The peristaltic pumping action is well suited for use in a medical infusion pump because it is applied externally to the fluid delivery tube and, consequently, does not interfere with the sterile state which must be maintained for the infusion fluid within the tube. Furthermore, the pumping action can be applied at any point on the delivery tube of a simple I.V. set, thus avoiding the need for special and expensive I.V. sets of the cassette type, as used on many volumetric infusion pumps.
There are various types of peristaltic pump, but the simplest, most reliable and least expensive is that based on a rotary action in which a rotary pump head, including a plurality of rollers, is rotated to engage the rollers successively with a delivery tube to pump fluid along the tube. This type of rotary pump mechanism is well known.
A major problem which exists in rotary peristaltic pumps is that the resulting delivery of fluid occurs in a series of pulses or surges, the frequency of which is equal to the frequency of the passage of successive rollers in contact with the delivery tube. This flow pattern is inherent in conventional rotary peristaltic pumps. The effect is that fluid is delivered at a widely varying rate during a pump cycle and this can be unacceptable in infusion procedures in which uniformity of delivery rate is a requirement. Moreover, the continuous change in flow rate can cause instability in sensitive feed-back control systems which are designed to ensure that fluid is delivered at a constant rate.
One convenient device for driving the pump head of a rotary peristaltic pump is a stepping motor, since such a motor is capable of offering a wide range of rotational speeds using a simple, gearless direct drive transmission. Also, the digital system used to rotate the motor is well suited to the type of electronic motor speed control circuitry used with a feed-back loop system for maintaining accuracy of pumping performance.
For the purposes of monitoring the flow pattern of fluid delivered by a typical peristaltic pump, tests were carried out on a rotary peristaltic pump arrangement having a rotary pump head comprising five equally spaced rollers and driven by a stepping motor having 200 steps per revolution. From these tests, it was found that during the passage of each roller in contact with the delivery tube, constant flow was maintained through the tube for 26 steps of the motor, immediately followed by a sequence of 14 motor steps during which there was no flow at all in the downstream or positive direction. During this dwell period, there was often some evidence of negative flow. This means that, in normal operation, the pump is delivering no fluid for over one-third of its operating time and is delivering fluid at a rate 50% higher than the average rate for nearly two-thirds of the time.